doctest::String ctor with non-zero terminated string

[Leandros]

Constructing a doctest::String from a StringView requires copying the result into a temp buffer first, before copying into a doctest::String.
std::string, for example, has a constructor which takes a char const * and size_t.

[onqtam]

I'll add such a constructor pretty soon :)

[Leandros]

Thanks!